NGFB will pickup and use all attached images to a Post or Page. Generally images are attached to a Post or Page in the context of using them in the content text, but images can also be attached directly from the Media Library listing, or by uploading new images when editing a Post or Page. If you find that an image is showing up in the Open Graph meta tags, that is not shown in the content, there’s a chance this image was attached and never detached from the Post or Page.
WordPress offers features to attach image, but not detach them (yes, it’s a little odd, I know). As a consequence of this, several plugins are now available to fill-in the missing features. Here are a few you might try: